Photoelectrochemical cells have been used to convert solar energy to hydrogen gas by splitting water into hydrogen and oxygen, hence offering the possibility of clean and renewable energy. Many photoelectrochemical cells have used titanium dioxide (TiO2), but the large band gap of TiO2 (about 3.1-3.3 eV) impedes the absorption of visible light and limits the solar-to-hydrogen efficiency to about 2.2%. So, it is necessary to use other materials that have a smaller band gap and can more efficiently harvest energy from sunlight.
There are many semiconductor materials with a lower band gap than TiO2, such as iron oxide (Fe2O3), bismuth vanadium oxide (BiVO4), tungsten oxide (WO3) and tantalum nitride (Ta3N5), for example. Alpha (α)-hematite, in particular, has a solar-to-hydrogen conversion efficiency of about 16%. Additionally, α-Fe2O3 has a low bandgap (2.1-2.2 eV), low cost, high chemical stability, nontoxicity, and natural abundance. It has several drawbacks as well, however, such as a relatively short hole diffusion length, low conductivity, shorter lifetime of photoexcitation, and deprived reaction kinetics of oxygen evolution. Some have tried doping with certain metals, such as titanium (Ti), molebdenum (Mo), aluminum (Al), zinc (Zn), platinum (Pt), and silicon (Si), for example, to improve the PEC performance of α-Fe2O3.